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Winter Gem boxwood (Buxus microphylla "Winter Gem") is one of many cultivars in the Buxus genus. It is hardy down to United States Department of Agriculture zone 5b, and it has leaves that are a deeper green than other varieties. Buxus species are generally hardy plants that adapt to a range of climatic zones, but they are susceptible to disease and problems that cause their leaves to yellow.

Normal Yellowing

Boxwood leaves can lose their green color during the winter if they’re grown in full sun. However, when temperatures warm again in the spring, they will regain their normal green color. Planting a tree nearby to shade your boxwood can prevent this problem, but it isn’t usually harmful to plant health. Boxwoods exposed to full sun may burn in hot temperatures during the summer months, and this can cause the tips to yellow as they burn. Lack of adequate sunlight and soggy soil conditions can also cause yellowing of the leaves. Marginal yellowing and etching of the leaves, with random sunken, tan-colored spots may also occur. These occur sporadically and do not harm your boxwood as long as it is otherwise healthy.

Phytophthora Root Rot

Winter Gem boxwoods are susceptible to phytophthora root rot (Phytophthora parasitica), which causes poor growth and foliage that is light green at first, turning yellow, straw-colored or bronze as infection progresses. Leaves may turn up and the lateral margins may roll inward, and the bark at the base of the boxwood typically dies. By the time you notice foliar symptoms, the roots have already decreased in number and many of these have turned brown, because the impaired root function precedes yellowing of the top portions of the plant. Boxwoods growing in water-logged soils caused by heavy rains or overwatering are more susceptible to phytophthora infection. Provide good drainage and cut back watering if infection appears. Phytophthora spores spread infection as they move with water. Foliar applications of the fungicide fosetyl-al may prevent the spread of infection.

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Root Nematodes

Infestations of plant parasitic nematodes can damage the roots of Winter Gem boxwoods, causing a slow decline that causes bronzing and yellowing of the leaves and eventually dieback. Nematodes feed on root cells. This not only damages the root system, but it also leaves your boxwood vulnerable to fungal infection. There are currently no effective methods of control for root nematodes on boxwoods. By providing proper cultural conditions such as adequate water, food and drainage, you can ensure your boxwood is better able to tolerate some nematode injury. Organic matter added to the soil can encourage antagonistic microorganisms that control nematodes. According to the University of California, there are currently no fumigants or nematicides available to home gardeners for nematode control.

Volutella Stem Blight

Volutella stem blight is associated with canker infections and wilting of boxwoods. It is also known as Pseudonectria canker and is caused by Pseudonectria rousseliana (conidial state of the Volutella buxi fungus). Symptoms of infection appear in the spring before the new growth forms. The leaves on the tips of twigs turn bronze or orange, and later straw-colored. The infected twigs die back, and cankers that are dark brown or black in color can be seen when the bark is cut away. Clusters of pink conidia may also be seen on the surface of lower leaves and branches. Infection may be controlled by pruning infected branches back to the healthy tissue. Spraying Bordeaux and fixed copper fungicides in the spring may help control the spread of volutella stem blight.

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About the Author

Renee Miller began writing professionally in 2008, contributing to websites and the "Community Press" newspaper. She is co-founder of On Fiction Writing, a website for writers. Miller holds a diploma in social services from Clarke College in Belleville, Ontario.